There is a great need in the semiconductor industry for metrology equipment which can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,636,088, 4,854,710 and 5,074,669. The latter patents are incorporated herein by reference.
The Therma-Probe device monitors ion implant dose using thermal wave technology. In this device, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample which spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way which alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface which alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused colinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal which is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes which are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
Thermal wave technology is well suited for measuring lattice damage in crystalline materials and, therefore, serves as an excellent technology for monitoring the ion implant process in semiconductor materials. It is also known that optical methods, such as spectroscopic reflectance and spectroscopic ellipsometry, are sensitive to lattice damage through the effects of such damage on the optical properties of the material being implanted.
Typically, thermal waves are more sensitive in the region of low implantation, i.e. less than 1012 ions/cm2 (arsenic at 30 KEV) than the optical methods. In the range of 1012 through 1014 ions/cm2, it appears that optical and thermal waves are comparable in their ability to detect changes in lattice damage. At higher doses (of the same implant), amorphization sets in and the thermal wave signal is no longer monotonic with increasing dose and cannot be used reliably to monitor the implant process. In this high dose region, the optical methods are very sensitive and can unambiguously measure the thickness of the amorphous layer.
There is, however, damage above and below the amorphous layer which can still make an accurate measurement of total damage in the implanted material difficult using only optical methods. More specifically, the implantation process at high doses will create a large damaged region with a relatively smaller layer of amorphous material in the center thereof. This occurs because during the implantation process, the ions travel very quickly as they first strike the lattice. The fast passage through the lattice can result in little or no damage immediately beneath the surface. As the ions begin to slow down, the damage increases until at a certain depth, the damage is sufficient to produce amorphization. Amorphization represents the peak damage to the lattice. Ions which travel beyond the amorphous layer will cause further damage, but below the threshold for amorphization. Semiconductor manufacturers are interested in knowing both the thickness of the amorphous region, as well as the total extent of damage to the lattice which would include the damaged regions both above and below the amorphous layer.
Thermal waves are intrinsically more sensitive to total damage than the optical methods. Therefore, by combining thermal waves with spectroscopic measurements, one can provide a means for sensitive and unambiguous monitoring of the ion implant process throughout the entire range. More specifically, one can use the data derived from the thermal wave measurements to provide an indication of the full extent of the damaged region. Data obtained from a spectroscopic measurements can be used to provide an indication of the thickness of the amorphous layer. By combining these two sets of measurements, one can provide an accurate profile of the damage as a function of depth below the surface of the semiconductor wafer.
The concept of combining thermal wave measurements with other optical measurements is disclosed in prior U.S. Pat. No. 5,978,074, issued Nov. 2, 1999, and is assigned to the same assignee as the subject invention and is incorporated herein by reference. This patent describes the need to obtain additional measurements where the sample is more complex. In one aspect of that patent disclosure, a conventional thermal wave detection system was modified to increase the amount of data which could be obtained. For example, a steering system was provided for varying the distance between the pump and probe beam spots as measurements were taken. Another approach was to obtain a sequence of measurements at various pump beam modulation frequencies.
The prior patent also discussed the advantages of combining spectroscopic reflectivity measurements with the thermal wave measurements. Various additional measurements were suggested including the assignee's proprietary beam profile reflectometry and beam profile ellipsometry techniques. The latter two approaches are described in U.S. Pat. Nos. 4,999,014 and 5,181,080, both of which are incorporated herein by reference.
The principal application for the tool described in U.S. Pat. No. 6,587,974 relates to measuring thin metal films formed on semiconductor samples. The latter patent did not disclose the advantages of combining thermal wave measurements with spectroscopic ellipsometry measurements. Further, the latter patent did not discuss the specific concept of using a thermal wave measurement to provide information on the full extent of a damage layer, while using another optical measurement to provide an indication of the amorphous layer.
Accordingly, it is an object of the subject invention to provide a new method and apparatus which provides additional measurement capabilities.
It is another object of the subject invention to provide a method and apparatus particularly suited to evaluating high dopant concentrations in semiconductors.
It is a further object of the subject invention to provide a method and apparatus which combines measurements of modulated optical reflectivity with modulated spectroscopic ellipsometry.